Method of obtaining concentrated potassium hydroxide in a colorless condition



' Sept. 29, 1970 I. FERRARA v METHOD OF OBTAINING CONCENTRATED POTASSIUM(2Q u XE m .r E mmmcoq Ira/o Ferrara INVENTOR.

UOOQM OOMN om 09 m ua om w v A mom: nmom how wowv \QtJ QQ UcOU IO BaniES Eabums StfltCS P2113111".

3,531,244 Patented Sept. 29, 1970 3,531,244 METHOD OF OBTAININGCONCENTRATED POTASSIUM HYDROXIDE IN A COLORLESS CONDITION Italo Ferrara,Siracusa, Italy, assignor t Sincat-Societa Industriale Catanese S.p.A.,Palermo, Italy, a corporation of Italy Original application Sept. 12,1966, Ser. No. 578,846, now Patent No. 3,436,178, dated Apr. 1, 1969.Divided and this application Sept. 26, 1968, Ser. No. 762,924 Claimspriority, application Italy, Sept. 14, 1965, 20,480/ 65 Int. Cl. C01dN42 US. Cl. 23-184 8 Claims ABSTRACT OF THE DISCLOSURE Method ofdecoloring potassium hydroxide after concentration thereof byevaporation, wherein a reducing agent from the group of alkali-metalmetabisulfites, hydrosulfites, sulfites and bisulfites (preferablysodium metabisulfites, sodium hydrosulfite or sodium bisulfite) is addedto the potassium hydroxide while the latter is at a temperature betweensubstantially 250 C. and 300 C. (preferably above 270 C.). Theproportions of sodium metabisulfite, sodium hydrosulfite, sodium sulfiteand sodium bisulfite preferably are in the following ranges:0.001-0.005%, 0.001-0.005%, 0.0020.02% and 0.005- 001%, respectively.

This application is a division of my copending application Ser. No.578,846, filed Sept. 12, 1966, which issued as U.S. Pat. 3,436,178 onApr. 1, 1969.

My present invention relates to a method of obtaining colorless highlyconcentrated potassium hydroxide and, more particularly, to improvementsin a process for producing high-titer potassium hydroxide.

In prior methods of producing concentrated potassium hydroxide(generated by electrolysis of KCl for example) and, eventually,potassium hydroxide titering approximately 90% or thereabouts, apotassium-hydroxide solution with a concentration of about 50% issubjected to continuous evaporation under vacuum in nickel concentratingvessels. The substantially pure liquid potassium hydroxide (titering 90%or more) emerges from the concentrators at a temperature slightly below300 C. (i.e., at the concentration temperature) and is poured into irondrums cooled externally by water. The resulting product has a more orless intense dark gray coloration which, I have found, is due to acorrosive action by the potassiumhydroxide solution upon the nickelwalls of the concentrating vessel and the formation of colored nickelcompounds.

It is, therefore, the principal object of the present invention toprovide an improved method of obtaining a substantially colorlesshigh-titer potassium hydroxide.

A further object of this invention is to provide a method, in a processfor producing potassium hydroxide involving the concentration of apotassium hydroxide solution in a nickel evaporator at elevatedtemperature, whereby a substantially colorless concentratedpotassiumhydroxide product can be obtained.

I found that, in accordance with the principal aspect of the presentinvention, the concentrated potassium hydroxide emerging from a nickelconcentrator can be clarified or rendered substantially colorless byaddition to the molten potassium hydroxide, prior to its introductioninto the iron drums (as is conventional in most cases) of smallquantities of a reducing substance capable of interacting with thenickel entrained in the emerging product liquid to produce colorlessreaction products.

Small quantities of at least one reducing substance selected from thegroup consisting of the alkaline-metal salts of citric and tartaricacids and especially the sodium and potassium salts thereof have provedto be effective as described and claimed in application Ser. No.578,846. Preferably sodium citrate and sodium potassium tartrate areused individually or in combination.

According to this invention the reducing substance is selected from thegroup of ionizable inorganic compounds containing metabisulphite (S 0hydrosulphite (S 0; sulphite (SO and bisulphite (HSO ions. Of thislatter class of compounds, the alkaline-metal salts and preferably thesodium or potassium salts of the metabisulphites, hydrosulphites,sulphites and bisulphites have proved to be most effective, thecompounds being used either individually or in mutual admixture.

Although the addition of the reducing substance may be carried out witha solid (e.g., pulverulent) reducing agent or a mixture thereof, it hasbeen found to be preferable to add an aqueous solution of the reducingsubstances to the concentrated potassium-hydroxide product at the outletof the nickel concentrator just prior to its introduction into the irondrums mentioned earlier. The reducing agent is added before the productenters the iron drums when the temperature of the liquidpotassiumhydroxide stream still exceeds 250 C. and is preferably between270 and 300 C.

The quantities of the reducing substances will, of course, depend uponthe concentration of the nickel in the potassium hydroxide so thathigher nickel contents will require correspondingly higher quantities ofthe reducing substances individually or together. For the commonpotassium-hydoxide concentrations of to 91% by weight, for example,which may have a nickel content between 0.0001 and 0.001% by weight, thequantity of reducing substance to be employed will preferably rangebetween 0.001 and 0.01% by weight with respect to thepotassium-hydroxide concentration calculated at In general terms,therefore, it can be stated that the *weight ratio of the reducingsubstancewhen the latter is a citrate or tartrate salt-4o the nickelconcentration will be of the order of 10 to 1. When the reducing agentsare of the sulphite class (e.g., sodium metabisulphite, sodiumhydrosulphite, sodium sulphite and sodium bisulphite), a similarrelationship is suitable.

More specifically, the proportion of sodium metabisulphite should rangebetween 0.001 and 0.005%, the concentration of sodium hydrosulphiteshould range between 0.001 and 0.005%, the concentration of sodiumsulphite should range between 0.002 and 0.02% and the concentration ofsodium bisulphite should range between 0.005 and 0.01% for thepotassium-hydroxide products mentioned.

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription, reference being made to the accompanying drawing, the solefigure of which represents a flow diagram illustrating the principles ofthe present invention, and the following group of specific examplesillustrating the inventive idea.

In the drawing, I show a potassium-hydroxide plant 1 whose productgenerally is a potassium-hydroxide solution of a concentration ofapproximately the order of 50%, this aqueous solution of potassiumhydroxide being fed to nickel concentrators 2, there to be subjected toevaporation under vacuum at temperatures of 250 to 300 C. The high-titerpotassium hydroxide 3 emerging from the nickel concentrators isintroduced into iron drums 4 which are shown diagrammatically to bewater-cooled in a bath at 5. In accordance with the principles of thisinvention, a solution of the reducing substance is added dropwise from adispenser 6 into the product stream 3 while the latter is at atemperature above 250 C. and usually of the order of 270 to 300 C. at arate determined by the concentration of nickel in the product.

EXAMPLE I A solution of sodium metabisulphite designed to provide 0.0025kg. of sodium metabisulphite per 100 kg. of potassium hydroxide (at100%) was dripped into potassium-hydroxide stream emerging from a nickelconcentrator with a KOH titer of 90.7%, a nickel content of 0.0009% anda temperature between 270 and 300 C., prior to any significant coolingof the concentrated caustic. A sample of the potassium-hydroxidesolution thus treated, when examined after cooling, was essentiallycolorless while a further sample not subjected to treatment with thereducing substance was of dark-gray coloration.

EXAMPLE II An aqueous solution of sodium metabisulphite calculated at0.004 kg. of sodium metabisulphite per 100 kg. potassium hydroxide (at100%) was dripped into a potassium-hydroxide stream emerging from anickel concentrator prior to its introduction into iron drums cooled ina water bath. The potassium-hydroxide stream had a KOH titer of 90.8%and a nickel content of 0.001%. The addition of the reducing solutionwas carried out when the potassium-hydroxide stream was at a temperaturebetween 270 and 300 C. The treated potassium hydroxide was found to becolorless, whereas the potassium hydroxide without treatment had adark-gray coloration.

EXAMPLE III An aqueous solution of sodium metabisulphite calculated toyield 0.002 kg. of sodium metabisulphite per 100 kg. of potassiumhydroxide at 100% was dripped into the potassium-hydroxide stream which,in this case, had a KOH titer of 91.0% and a nickel content of 0.0003%.Again the temperature of the potassium-hydroxide stream ranged between270 and 300 C. as it left the nickel concentrators and was treated withthe reducing agent. The treated product was substantially colorlesswhereas the product prior to treatment with the reducing agent had adark-gray coloration.

EXAMPLE IV A solution of reducing agent was prepared in such manner asto obtain 0.0017 kg. of sodium metabisulphite per 100 kg. of KOH (at100%) and dripped into potassium hydroxide having a titer in KOH of90.6% and a content in Ni of 0.0001%.

The addition of this solution was carried out immediately prior to thefilling of iron drums with the potassium hydroxide, when the latter wasat a temperature between 270 and 300 C., emerging from a nickelconcentrator.

As sample thus treated, when examined after cooling, appeared to becolorless, while a sample of the same potassium hydroxide but notsubjected to the treatment with the reducing substance had a darkgraycoloring.

EXAMPLE V A solution of sodium hydrosulphite designed to yield 0.002 kg.of sodium hydrosulphite per 100 kg. of KOH (at 100% was added dropwiseto potassium hydroxide having a titer of 90.5% KOH and a nickel contentof 0.0008%.

The addition of this solution was carried out immediately prior to theintroduction of the potassium hydroxide into the drums, when thepotassium hydroxide was at a temperature between 270 and 300 C.

A sample thus treated, when examined after cooling, appeared to becolorless, while another sample of the same potassium hydroxide, but notsubjected to the treatment with the reducing substance, appeared to beof a dark-gray color.

EXAMPLE VI A solution of sodium sulphite calculated so as to obtain0.003 kg. of sodium sulphite per 100 kg. of KOH (at 100% was drippedinto potassium hydroxide having a KOH titer of 90.7% and a nickelcontent of 0.0008%.

The addition of this solution was carried out immediately prior to theintroduction of the potassium hydroxide into the drums when thepotassium hydroxide was at a temperature between 270 and 300 C.

A sample thus treated, when examined after cooling, appeared to becolorless, while another sample of the same potassium hydroxide, but notsubjected to the treatment with the reducing substance, appeared to bedark gray.

EXAMPLE VII A solution of sodium bisulphite calculated so as to contain0.005 kg. of sodium bisulphite per 100 kg. of KOH (at 100% was feddrop-by-drop to potassium hydroxide having a titer of 91.0% KOH and acontent in Ni of 0.0007%.

The addition of this solution was carried out immediately prior to thefilling of iron drums with the potassium hydroxide, while it still wasat a temperature between 270 and 300 C.

A sample thus treated, when examined after cooling, appeared to becolorless, while another sample of the same potassium hydroxide, but notsubjected to the treatment with the reducing substance, was of dark-graycoloration.

When potassium salts of the reducing ions were substituted for thesodium salts, and when the reducing salts were added in the powderedstate instead of in solution, substantial decoloration of theconcentrated KOH was also obtained.

I claim:

1. In a process for producing concentrated potassium hydroxide of a highKOH titer wherein a potassium-hydroxide solution is subjected toconcenu'ation by evaporation at an elevated temperature in a nickelconcentrating vessel whereby a coloration is imparted to theconcentrated potassium hydroxide emerging from said vessel, theimprovement which comprises the step of adding to the concentratedpotassium hydroxide emerging from said vessel, while it is at atemperature between substantially 250 and 300 C., at least one reducingagent capable of interacting with nickel compounds entrained in theemerging potassium-hydroxide stream to form substantially colorlessproducts therewith, said reducing agent being selected from the groupwhich consists of ionizable inorganic metabisulphites, hydrosulphites,sulphites and bisulphites.

2. The improvement defined in claim 1 wherein the potassium hydroxideemerging from said vessel is at a temperature between substantially 250and 300 C. and is introduced into cooled drums, said reducing agentbeing added to the potassium hydroxide prior to its introduction intosaid drums.

3. The improvement defined in claim 2 wherein said reducing agent isadded to said potassium hydroxide emerging from said vessel while it isat a temperature between 270 and 300 C.

4. The improvement defined in claim 1 wherein the potassium hydroxideemerging from said vessel titers at least KOH and contains between0.0001 and 0.001% by weight nickel, said reducing agent being added tothe potassium hydroxide emerging from said vessel in an amount rangingbetween 0.001 and 0.01% by weight of the reducing agent based upon apotassium-hydroxide concentration of 5. The improvement defined in claim4 wherein said reducing agent is an alkali-metal metabisulphite,hydrosulphite, sulphite or bisulphite.

6. The improvement defined in claim 5 wherein said reducing agentincludes at least one compound from the group of sodium metabisulphite,sodium hydrosulphite, sodium sulphite and sodium bisulphite.

7. The improvement defined in claim 5 wherein said reducing agent isadded to the potassium hydroxide in solution.

8. A method of clarifying a colored concentrated potassium hydroxidetitering at least 90% KOH and containing nickel upon its emergence froma nickel concentrating vessel at a temperature between 270 and 300 0,comprising the steps of:

adding to the potassium hydroxide at said temperature a reducing agentfrom, the group of alkali-metal 6 metabisulphites, hydrosulphites,sulphites, and bisulphites; and thereafter cooling the potassiumhydroxide.

References Cited UNITED STATES PATENTS 2,562,169 7/1951 Brumbaugh 23-1842,735,750 2/1956 Rahn 2 s 23-184 2,889,204 6/1959 Meyer et al 23184 103,325,251 6/1967 Akker 23-184 EDWARD J. MEROS, Primary Examiner

